This invention relates generally to inflators such as for use in inflating inflatable restraint airbag cushions to provide impact protection to occupants of motor vehicles. More particularly, the invention relates to inflators which rely primarily on reaction of a combustible material for the production of an inflation gas and such as may provide a gas flow orifice for adaptive inflation gas output.
It is well known to protect a vehicle occupant using a cushion or bag, e.g., an xe2x80x9cairbag,xe2x80x9d that is inflated or expanded with gas when the vehicle encounters sudden deceleration, such as in the event of a collision. In such systems, the airbag cushion is normally housed in an uninflated and folded condition to minimize space requirements. Upon actuation of the system, the cushion begins being inflated in a matter of no more than a few milliseconds with gas produced or supplied by a device commonly referred to as an xe2x80x9cinflator.xe2x80x9d
Various types of inflator devices have been disclosed in the art for the inflation of an airbag such as used in inflatable restraint systems. One type of known inflator device derives inflation gas from a combustible pyrotechnic gas generating material which, upon ignition, generates a quantity of gas sufficient to inflate the airbag.
In general, the burn rate for a gas generant composition can be represented by the equation (1), below:
rb=k(P)nxe2x80x83xe2x80x83(1)
where,
rb=burn rate (linear)
k=constant
P=pressure
n=pressure exponent, where the pressure exponent is the slope of a linear regression line drawn through a log-log plot of burn rate versus pressure.
As will be appreciated, the pressure exponent generally corresponds to the performance sensitivity of a respective gas generant material, with lower burn rate pressure exponents corresponding to gas generant materials which desirably exhibit corresponding lesser or reduced pressure sensitivity.
Typical pyrotechnic-based inflator devices commonly include or incorporate certain component parts including, for example: a pressure vessel wherein the pyrotechnic gas generating material is burned; various filter or inflation medium treatment devices to properly condition the inflation medium prior to passage into the associated airbag cushion; and a diffuser to assist in the proper directing of the inflation medium into the associated airbag cushion.
To date, sodium azide has been a commonly accepted and used gas generating material. While the use of sodium azide and certain other azide-based gas generant materials meets current industry specifications, guidelines and standards, such use may involve or raise potential concerns such as involving handling, supply and disposal of such materials. Further, economic and design considerations have also resulted in a need and desire for alternatives to azide-based pyrotechnics and related gas generant materials. For example, interest in minimizing or at least reducing overall space requirements for inflatable restraint systems and particularly such requirements related to the inflator component of such systems has stimulated a quest for gas generant materials which provide relatively higher gas yields per unit volume as compared to typical or usual azide-based gas generants. Still further, automotive and airbag industry competition has generally lead to a desire for gas generant compositions which satisfy one or more conditions such as being composed of or utilizing less costly ingredients or materials and being amenable to processing via more efficient or less costly gas generant processing techniques.
As a result, the development and use of other suitable gas generant materials has been pursued. Through such efforts, various azide-free pyrotechnics have been developed for use in such inflator device applications including at least some which have or exhibit a relatively high burn rate pressure dependency, e.g., have a burn rate pressure exponent of 0.4 or more, at 1000 psi.
Typical pyrotechnic-based inflators involve the reaction of a gas generant to form an inflation gas which is released from the inflator device to effect the desired inflation of an associated airbag cushion. The rate at which inflation gas is produced or formed in an inflator is typically a significant factor in the rate at which an associated airbag cushion is inflated. While a rapid or high inflation rate is generally required in order to achieve inflation and deployment of an associated airbag cushion in order to provide desired vehicle occupant protection, efforts have been directed to reduce the mass flow rate of inflation gases into the airbag cushion during the initial stages of deployment such as to minimize or avoid the risk of injury to a vehicle occupant who are out of the desired traveling position (with such vehicle occupants often referred to as xe2x80x9cout of position occupantsxe2x80x9d).
Airbag installations providing a slower initial deployment rate, also referred to as low onset inflation, followed by an increased deployment rate can have the benefit of providing a more gradual initial deployment of the associated airbag cushion into the occupant-containing vehicle compartment yet still achieve desired full or complete inflation within the desired time frame. Current low onset inflation is generally best achieved via two-stage inflator devices. However, two-stage inflators commonly require two electrical initiators and are generally more expensive than single stage inflator devices.
Methods of obtaining low onset inflation via single stage inflators have generally not provided the desired deployment rate curve. Such single-stage inflator methods include: inhibiting the surface of the gas generant such as by coating or otherwise covering a surface portion or side of a gas generant tablet; initially cooling the inflation gasses in a heat sink that saturates quickly, wherein the saturated heat sink will no longer cool the gasses resulting in an increased pressure; methods for altering generant grain shape; and other methods that alter the ignition conditions to provide a non-synchronous ignition of all gas generant material.
In view of the above, there is a need and a demand for improved arrangements and methods for providing low onset inflation of airbag cushions, particularly with single stage inflator devices such as employ only a single electrical initiator. Further, there is a need and a demand for combustible material-based inflator devices which provide or result in a slower initial rate of deployment followed by an increase in deployment rate. Further, there is a need and a demand for such an inflator device which more freely permits the use of azide-free pyrotechnics, such as those which have or exhibit a relatively high burn rate pressure dependency. Still further, there is a need and a demand for such a low onset inflator device that is less costly to manufacture or produce. Yet still further, there is a need and a demand for single stage inflator devices that provide or result in low onset inflation without requiring the inclusion of complex or costly control devices or arrangements.
A general object of the invention is to provide an improved inflator and associated or corresponding methods of supplying inflation gas.
A more specific objective of the invention is to overcome one or more of the problems described above.
The general object of the invention can be attained, at least in part, through an improved inflator device having at least one orifice wherethrough inflation gas can pass. In accordance with one preferred embodiment of the invention, the at least one orifice is at least in part defined by a shape memory alloy material having an austenite finishing temperature (Tf), wherein the at least one orifice defines a first fluid flow through area (A1) when at a temperature less than Tf and a second fluid flow through area (A2) when at a temperature greater than Tf, where A2 less than A1.
As described in greater detail below, shape memory alloys in accordance with the invention can be initially formed into a first shape and subsequently deformed or stressed into a second shape while in a martensite phase. When heated to a temperature where the shape memory alloy forms the austenite phase, referred to as the austenite finishing temperature (Tf), the shape memory alloy generally returns to the prestressed or unmodified martensite shape.
In accordance with a preferred practice of the invention, adaptability in inflator output is achieved through change in cross sectional area of the orifice such as to result in a change in combustion pressure. In particular, practice of the invention in conjunction with a gas generant material, e.g., pyrotechnic, having a burn rate which is pressure dependant as herein defined, results in changes in combustion pressure correspondingly changing the burn rate of the gas generant, thus altering or adapting the inflator output, e.g., inflation gas mass flow rate. For example, reducing the inflator orifice area raises the combustion pressure within the gas production chamber which, in turn, raises the burn rate of the gas generant material which increases the inflation gas mass flow rate from the inflator. Correspondingly, increasing the inflator orifice area reduces the combustion pressure within the gas production chamber which, in turn, reduces the burn rate of the gas generant material which decreases the inflation gas mass flow rate from the inflator. Such performance behavior is opposite to that of at least certain prior art inflator devices such as certain stored gas inflators which incorporate an adjustable exit area. In particular, such prior art inflator devices typically experience a reduction in inflation gas mass flow rate with a reduction in exit area and an increase in inflation gas mass flow rate with an increase in exit area.
The prior art generally fails to provide inflator devices with low onset inflation that are of as simple a design and construction as may be desired. In particular, the prior art fails to provide such a low onset inflator device which relies largely or primarily on the reaction of a combustible material, e.g., a pyrotechnic, especially such as various azide-free pyrotechnics which have or exhibit a relatively high burn rate pressure dependency, to form or produce inflation gas. Further, the prior art generally fails to provide adaptive performance inflatable restraint assembly combinations which incorporate shape memory alloy technology to change or alter the internal pressure of the combustion chamber thereby increasing gas mass flow rate as the gas generant reacts.
The invention further comprehends an airbag inflator device with a first chamber wherein a supply of a combustible gas generant material reacts to produce gas and an orifice assembly in fluid communication with the first chamber. The orifice assembly defines at least one orifice wherethrough at least a portion of the produced gas can pass. The orifice assembly includes at least one inflator device opening and a restrictor disposed adjacent the at least one opening. The restrictor device is at least in part defined by a shape memory alloy material with an austenite finishing temperature (Tf). The at least one orifice defines a first fluid flow through area (A1) when at a temperature less than Tf and a second fluid flow through area (A2) when at a temperature greater than Tf. The second fluid flow through area (A2) is less than the first fluid flow through area (A1) allowing for adaptability in inflator output.
The invention still further comprehends a self-regulating inflation gas rate flow inflator device with a first chamber for burning a supply of a combustible gas generant material having a burn rate which is pressure dependent to form a product gas and at least one orifice wherethrough at least a portion of the product gas can pass. The at least one orifice is preferably at least in part defined by a shape memory alloy material comprising a ternary alloy including copper, aluminum and one of nickel and bromine. The shape memory alloy has an austenite finishing temperature (Tf) of at least 90xc2x0 C. and the at least one orifice defines a first fluid flow through area (A1) when at a temperature less than Tf and a second fluid flow through area (A2) when at a temperature greater than Tf. The second fluid flow through area (A2) is less than the first fluid flow through area (A1) allowing for adaptability in inflator output.
As used herein, references to a xe2x80x9cshape memory alloyxe2x80x9d are to be understood to refer to metal alloys characterized by the ability to be quickly restored to a prestressed shape at a predetermined temperature that causes a change from a martensite phase to an austenite phase. Shape memory alloys can be formed into a first shape and then stressed into a second shape while in the martensite phase. Upon heating the alloy material to the austenite phase, the alloy is suitably returned to the prestressed martensite shape.
As used herein, references to xe2x80x9caustenite finishing temperaturexe2x80x9d generally refer to the temperature at which the martensite to austenite reaction is completed upon heating.
As used herein, references to xe2x80x9cself-regulatingxe2x80x9d inflation gas flow inflator devices are to be understood to generally refer to those inflator devices which require no external sensors or other control equipment to adjust the gas flow from the inflator device to an associated airbag cushion. Correspondingly, the xe2x80x9cself-regulatingxe2x80x9d function of shape memory alloys in accordance with a preferred embodiment of the invention is dependant on predetermined temperatures and therefore desirably requires no additional sensors or control equipment.
Further, references herein to a combustible gas generant material, e.g., a pyrotechnic, having a burn rate which is xe2x80x9cpressure dependentxe2x80x9d are to be understood to refer to those combustible gas generant materials having a relatively high burn rate pressure dependency. In the context of the invention, such a relatively high burn rate pressure dependency is generally signified by a burn rate pressure exponent of at least about 0.4 at 1000 psi, preferably by a burn rate pressure exponent in the range of about 0.4 to about 0.6, at 1000 psi.
Other objects and advantages will be apparent to those skilled in the art from the following detailed description taken in conjunction with the appended claims and drawings.